PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 631 



branch of the carotid artery which Ues some distance lateral and 

 dorsal to it. In color, and general form and size, these cells are 

 well differentiated from the surrounding mesenchyme on the one 

 hand; and from the pharyngeal cells from which they have had 

 their origin on the other. These differences in color and shape 

 are interesting, since Miss Piatt ('96), in her description of this 

 stage in Necturus remarks that, "they are distinguished from 

 the surrounding mesenchyme merely by their closer grouping." 

 Maurer ('88) notes the condition of the thyreoid cells in Triton in 

 this stage in the following terms (p. 362), 



So lange die Zellen der Schilddriise Dotterbliittchen enthalten ist das 

 Organ leicht zu erkennen. Zwei bis dreiWochen nach dem Verlassen 

 des Eies verschwinden die Dotterblattchen und est halt dann eine Zeit 

 lang schwer, die Schilddriise aufzufinden. Sie besteht jederseits aus 

 einem wenig vortretenden soliden Zellschlauch, der an dem bezeich- 

 neten Platze, vor dem ersten Arterienbogen, gerade hinter dem zweiten 

 Keratobranchiale, zur Seite des Sternotiyoideus liegt. 



E. Ambly stoma larvae, 15 mm. long. Aside from the continued 

 growth of the surrounding parts, but little change is noted in the 

 relationships of the thyreoid cells from that of the 13 mm. stage. 

 Although distributed over practically the same area and the 

 same region of the body (occurring for 380 micra along the 

 inferior jugular vein, and extending from the point of junction of 

 the first branchial cartilage with the copula to the region of the 

 anterior wall of the pericardium) , the cells have increased in num- 

 bers and are more compact; a few (three to six) in a group, and 

 several groups usually being cut in the same section. This ar- 

 rangement of the cells in groups is the first indication of the forma- 

 tion of follicles, which are developed rapidly in the stages immedi- 

 ately following this. 



Contradictory statements are made as to the exact manner in 

 which the thyreoid follicles are developed from the primary 

 anlage. W. Miiller ('71), claimed that the thyreoid gland in all 

 vertebrates passes through three stages: (1) a severing of the 

 anlage from the pharynx ; (2) the formation of a network of tubes 

 of glandular epithelium; and (3) the formation of follicles from 

 these tubes. Stockard ('06), in Bdellostoma and Gudernatsch 



